For many years, so-called solid tantalum capacitors have set the market standard for high capacitance per unit volume combined with high reliability. Since their introduction in the 1950's, solid tantalum capacitors have continued to shrink in size due to the introduction of tantalum powders having higher surface area per unit weight (i.e., smaller particle size). High surface area tantalum powders facilitate the use of smaller anodes having the same capacitance when anodized to equivalent anodic oxide thickness compared with older tantalum powders.
The utility of tantalum capacitors has been extended by the widespread introduction of surface mount solid tantalum capacitors in the 1980's. The heat-resistance properties inherent to solid tantalum capacitors due to the manganese dioxide counter electrode material used in the fabrication renders tantalum capacitors relatively immune to the destructive effects of heating during reflow-soldering compared with aluminum electrolytic capacitors which contain a liquid, organic-solvent-based electrolyte. Consequently, a large fraction of the applications which formerly utilized miniature aluminum electrolytic capacitors have been converted to solid tantalum capacitors of the surface mount configuration.
Further extending the utility of solid tantalum capacitors has been the introduction of inherently conductive polymers as counter electrode materials in place of the manganese dioxide traditionally present in these devices. The high electrical conductivity of inherently conductive polymers gives rise to a significant reduction in equivalent series resistance and loss of capacitance at higher frequencies in solid tantalum capacitors containing them. Solid tantalum capacitors with conductive polymer cathodes have the additional advantage of being resistant to ignition in the event of a short circuit occurring within the capacitor.
The recent introduction of surface mount, solid tantalum capacitors having multiple anode elements in parallel to reduce the equivalent series resistance of the devices to well under 10 milliohms further extends the use of solid,tantalum capacitors to applications where previously only ceramic or metallized film capacitors could be used. The tantalum capacitors are generally much smaller than the ceramic or metallized film capacitors which they replace.
The improvements in tantalum capacitors, described above, combined with the explosive growth in the computer and mobile telephone industries have resulted in the growth in worldwide demand for tantalum capacitors from a few million pieces per year in the 1950's to well over a billion pieces per month today. In spite of the improvements in surface area per unit weight made by the suppliers of tantalum capacitor powder over the years, the demand for tantalum for capacitor purposes has grown steadily since the 1950's. Tantalum is a relatively rare element in nature, and this fact coupled with increasing demand, has resulted in a forty-fold or more increase in the price of tantalum powder over the past forty to fifty years.
It is widely recognized that the growth of the electronics industry is driven by greater device performance at lower cost, as time advances. Thus, while the capacitance per unit volume continues to increase, the price per unit of capacitance for solid tantalum capacitors continues to decrease with time, as it must in order for these devices to conform to the so-called learning curve of device manufacturing cost versus the logarithm of the cumulative number of devices sold worldwide. This learning curve of the cost requirements for components must be satisfied in order to maintain the growth rate of the electronics industry.
It is widely recognized that, in spite of the device manufacturing cost learning curve, surface area per unit weight of tantalum cannot be increased indefinitely. It is also recognized that the increasing demand for tantalum is forcing tantalum producers to process lower quality ores in order to meet demand for the metal. The extraction cost per pound of tantalum increases significantly with decreasing ore quality.
In an effort to reduce the cost of the valve metal component of solid capacitors, other valve metals in addition to tantalum have been tested for use in solid capacitor manufacture. The metal, niobium, is most closely related to tantalum in chemical and physical properties. For many years, attempts have been made to fabricate successful solid capacitors from niobium powder. Early niobium powders contained a relatively large amount of impurities and gave rise to highly flawed oxide during anodization at the temperatures normally used to anodize tantalum anodes (i.e., 80° C. to 90° C.) in dilute phosphoric acid. It was found that the production of blister-like flaws in the anodic oxide on niobium could be minimized through the use of anodizing temperatures below about 25° C. Unfortunately, solid niobium capacitors were found to give increasing leakage current and shorted devices upon testing under voltage at elevated temperatures (e.g., 85° C.).
Niobium powders prepared recently appear much improved with respect to impurity content and may be anodized at traditional anodizing temperatures (i.e., 80° C. to 90° C.) with the production of relatively flaw-free oxide at low anodizing voltages (60 volts or less). Particularly good dielectric properties, as indicated by wet-cell testing of anodized niobium anodes, are obtained through the use of the electrolytes and methods described in co-pending application Ser. No. 09/143,373 and Ser. No. 09/489,471.
Unfortunately, even solid capacitors manufactured from relatively pure niobium powder are subject to increasing leakage current and short circuit failures on life test at elevated temperature. The failures have been traced to the migration of oxygen from the anodic oxide into the niobium substrate. This failure mechanism is also known in tantalum capacitors, but the effect is much more pronounced with niobium.
Fortunately, a solution to the problem of oxygen migration from the anodic film to the valve metal substrate has been found and has been demonstrated for tantalum and niobium. On Mar. 9, 2000, at the 20th Capacitor And Resistor Technology Symposium, Dr. Terrance Tripp presented a paper, entitled: “Tantalum Nitride: A New Substrate for Solid Capacitors” (Authors: T. Tripp, R. Creasi, B. Cox; reprinted in the symposium proceedings, on pages 256-262). This paper describes the anodic oxide-to-valve metal substrate thermally-driven oxygen migration problem for the tantalum-tantalum oxide system. The authors also describe a method of overcoming this problem via the substitution of tantalum nitride or sub-nitride for tantalum powder in the fabrication of device anodes (the tantalum nitride, TaN, actually loses half of its nitrogen content during the vacuum sintering step used to consolidate the powder into an anode body, becoming tantalum sub-nitride, Ta2N, by the end of the sintering process).
The presence of nitrogen in the tantalum sub-nitride substrate material does not appear to interfere with the formation of the anodic oxide film dielectric. Anodes prepared by vacuum sintering tantalum nitride or sub-nitride may be anodized in the electrolytes traditionally used in the tantalum capacitor industry as well as the electrolytes described in co-pending application Ser. No. 09/143,373 and Ser. No. 09/489,471 as well as PCT No. WO 00/12783. The anodic oxide films produced on sintered tantalum nitride or sub-nitride have proven to be greatly improved with respect to resistance to thermally driven oxygen migration, oxide-to-substrate, compared with anodic oxide films grown upon tantalum metal. This enhanced thermal stability toward oxygen migration is clearly demonstrated in the capacitance versus voltage bias curves for anodized and heat-treated tantalum and tantalum nitride substrates, depicted in FIG. 6 of the paper by Tripp, et. al.
Tripp, et. al., have extended their treatment of valve metals to niobium and have reported the same fundamental increase in resistance to oxygen migration for anodic oxide films on niobium nitride/sub-nitride compared with anodic films on niobium metal anodes as they observe for anodic films on tantalum/sub-nitride compared to anodic films on tantalum metal anodes. FIG. 9 of their C.A.R.T.S. paper (page 261 of the symposium proceedings) depicts the capacitance versus bias voltage for heat-treated anodic oxide films on niobium and on niobium nitride substrates. The improvement in stability for the anodic film on the niobium nitride is marked.
Fife, U.S. Pat. No. 6,051,044, discloses that at least some improvement in thermal stability of the niobium/niobium oxide system is realized by the presence of nitrogen in the niobium substrate at levels considerably below those present in the nitride or sub-nitride. Fife specifies as low as 300 ppm nitrogen and, more particularly, 300 ppm to 5000 ppm nitrogen (claim 4) constitutes an improvement over prior art niobium capacitor powders.
The mechanism by which the presence of nitrogen in the valve metal substrate gives rise to greater thermal stability of the anodic oxide/valve metal substrate with respect to oxygen diffusion into the substrate appears to be twofold. Nitrogen present at relatively low concentrations appears to act as a diffusion barrier, increasing the temperature and/or time required for degradation of the electrical leakage properties of the anodic oxide in a manner similar to the stability enhancement observed with tantalum anodes anodized in electrolyte solutions containing a relatively high concentration of orthophosphate ion, e.g., 1 to 5 wt. % or more orthophosphate.
The second mechanism through which the presence of nitrogen in the niobium or tantalum substrate gives rise to greater thermal stability of the anodic oxide/valve metal interface with respect to oxygen diffusion, particularly when the nitrogen is present in relatively large quantities, is the lowering of the chemical potential of the oxygen diffusion process (i.e., reducing the free energy liberated by the diffusion process) through the formation of relatively stable compounds of nitrogen and the valve metal substrate. Metal nitrides and sub-nitrides have a sufficiently high activation energy for decomposition that the temperatures employed for life-testing are many hundreds of degrees centigrade below those required for valve metal nitride decomposition.
The effectiveness of reducing the chemical potential driving oxygen diffusion (as opposed to employing an oxygen diffusion inhibitor) is illustrated in PCT No. WO 00/15555. This patent describes the use of a class of niobium suboxide, approximating NbO, in place of niobium for the fabrication of powder metallurgy capacitor anodes. The use of niobium suboxides as capacitor materials is said to result, not only in the production of anodes having greater resistance to surface area loss during sintering, hence greater capacitance retention, but also anodes which demonstrate a low level of electrical leakage current, presumably due to the inhibition of oxygen migration due to the reduction in chemical potential for the anodic oxide/niobium suboxide system versus the anodic oxide/niobium metal system.
Although the substitution of niobium for tantalum provides for a large potential reduction in material cost due to the more plentiful supply of niobium in nature (approximately 10 to 20 times more plentiful than tantalum), the lower density of niobium (8.57 gm/cc), compared with tantalum (16.6 gm/cc), and an extremely large deposit at a site in Brazil, there exist other materials which offer the possibility for even greater savings than niobium.
For many years it has been recognized that titanium forms an oxide having a very high dielectric constant (approximately 85) compared with tantalum oxide (approximately 26) and niobium oxide (approximately 41). Titanium has a relatively low density (4.54 gm/cc) and is very common in nature, being the 9th most common element in the earth's crust.
The production of adherent, relatively flaw-free and electrically insulating films on titanium is quite difficult due to the sensitivity of the material to anodizing electrolyte temperature, water content, proticity, as well as chloride contamination of both electrolyte and substrate metal. A process for successfully anodizing titanium and its alloys was developed in the early 1980's and is described by Melody in British Patent Application No. GB 2168383A. The method of this patent application is also described by Rosenberg, et. al., in the paper entitled: “Anodizing Mechanism in High Purity Titanium” (presented at the “Titanium '92,” 7th International Conference on Titanium, San Diego, Calif., 1992).
Unfortunately, although high quality dielectric films can readily be grown on titanium anodes using the methods of British Patent Application No. GB 2168383A, it has been found that anodic oxide films on titanium are subject to oxygen diffusion from the anodic oxide into the metal substrate even at room temperature. The solid solubility of oxygen in titanium is very large (almost 25 wt. %) as are the free energies of formation for the titanium suboxides. Thus titanium solid capacitor anodes are even more sensitive to thermal degradation than niobium solid capacitor anodes.
In much the same manner as niobium, titanium forms a nitride, TiN, and a series of suboxides. These materials possess high electrical conductivity and relative chemical inertness. While it is apparent that anodes fabricated from titanium nitride or suboxides should give rise to much more thermally stable anodic oxide films for the same reasons found with tantalum or niobium, unfortunately these materials give rise to oxygen evolution when biased positive in aqueous solutions and do not give rise to barrier-type anodic oxide films (or do so with such extremely low efficiencies that, for all practical purposes, they remain anodic oxide film-free).
In order to test this type of material, anode compacts were pressed from titanium nitride powder obtained from the GFS Chemical Company. These anode compacts were pressed to close to the theoretical density for the material in order to minimize current flow to any internal surfaces of the sintered anodes in order to facilitate study of current flow through the relatively small surface area represented by the envelope surrounding the anode bodies. The anode compacts were equipped with embedded tantalum wires for electrical contacts. The tantalum nitride anode compacts were then sintered at 1,600° C. for 20 minutes to produce consolidated titanium nitride anode bodies at near the theoretical density for the material.
It is noted that for capacitor production purposes, powder metallurgy valve metal anode compacts are generally produced at ⅓ to ⅔ of the theoretical density of the valve metal in order to make use of the large internal surface area of the anodes. The titanium nitride anode bodies, described above, were prepared for testing purposes only in order to study the anodizing behavior of the consolidated material, as one would study the behavior of foil coupons, without the complication of phenomena, such as gas evolution, arising from the reactions within the fine pores of traditional porous anodes.
The anodic film-forming properties of the full-density titanium nitride anodes were then determined in dilute phosphoric acid by suspending an anode in a stainless steel beaker of 0.1% phosphoric acid at 85° C. and applied d.c. voltage with the anode biased positive. As soon as the hydrogen reduction potential was reached, hydrogen was evolved at the stainless steel beaker and oxygen at the anode surface. There was no sign of anodic film formation, and increasing the applied voltage was found to increase the current; the current did not decay with time as is the case during anodic oxide film growth.
The gassing and lack of anodic film formation in this experiment are similar to (but even more pronounced than) the results obtained by anodizing titanium anodes in hot, dilute acids. In light of the successful results obtained anodizing titanium by employing the electrolytes and methods described in British Patent Application No. GB 2168383A, a full-density titanium nitride anode was immersed in a 25° C., 10 vol. % solution of 85% phosphoric acid in N-methyl-2-pyrrolidone (one preferred embodiment of GB 2168383A) contained in a stainless steel beaker. It was found that the titanium nitride anode could be readily anodized to well over 100 volts at a constant current of approximately 5 milliamperes per square centimeter. An additional titanium nitride anode body (full density) was anodized to 100 volts, again at approximately 5 milliamperes per square centimeter. Upon reaching 100 volts, the current decayed with time to a value less than 1% of the initial value, as is the case with tantalum, aluminum and other valve metal materials used to fabricate electrolytic capacitors. Thus the ability to anodize derivatives of valve metals which have been found to be difficult or impossible to anodize in aqueous electrolytes, such as the nitrides, and suboxides of titanium, etc., through the use of the electrolytes and methods of GB 2168383A was demonstrated.
Although the electrolytes and methods of GB 2168383A represent a major advancement over prior art, these methods and electrolytes have certain shortcomings from a manufacturing standpoint. The polar, aprotic solvents of this patent are difficult to contain in anodizing tank plumbing systems. It is also very difficult to maintain the water content of the electrolytes of GB 2168383A below 2 wt. % while anodizing at 30° C. or below without air-tight anodizing tank covers, vacuum-treatment to reduce the electrolyte water content, etc. The anodized anodes must be carefully transferred from the anodizing tanks to rinse systems so as not to allow the electrolytes of GB 2168383A to drip onto floors, equipment, or personnel due to the aggressive solvent action of these electrolytes (the electrolytes present tend to attack flooring and machine finishes, as well as operator clothing).
There also exist technical/process disadvantages with the electrolytes/methods of GB 2168383A. The resistivity of the electrolytes of this patent tend to exhibit excessively high resistivities at the anodizing temperatures where these electrolytes have been found useful (i.e., below approximately 30° C.), generally in excess of 1,000 ohm-cm at 1 kHz. The rapid and thorough anodizing of powder metallurgy anodes fabricated from finely powdered valve metals and designed for low voltage applications has been found to require electrolyte resistivities, at anodizing temperatures, of less than about 500 oh-cm at 1 kHz, and preferably on the order of 250 ohm-cm or less.
The aproticity of the electrolyte solvents of GB 2168383A (necessary to minimize adverse reactions during anodizing of very reactive substrates, such as titanium metal) presents an additional problem in the anodizing of porous anodes fabricated from finely powdered materials. The orthophosphate ion tends to accumulate within the anode bodies due to electrostatic attraction, with the cations (assuming amine salts are used to obtain minimum electrolyte resistivity) accumulating near the cathode surfaces. The orthophosphate ions within the anode bodies react with the low water content of the electrolytes to form orthophosphoric acid. Orthophosphoric acid is poorly ionized in aprotic solvent solutions and, in consequence, the resistivity of the electrolyte within the anode bodies rises to high levels, resulting in incomplete internal oxide formation unless very long anodizing times are employed.